Prepregs and laminates having homogeneous dielectric properties

ABSTRACT

Prepregs and laminates made from resin compositions having a free resin portion and a resin impregnated reinforcing material portion where the resin includes one or more base resins and one or more high Dk materials wherein the one or more high Dk materials are present in the resin composition in an amount sufficient to impart the resin composition with a cured Dk W  that matches the Dk WR  of a resin impregnated reinforcing material to which the resin composition is applied to within plus or minus (±) 15%.

This is a continuation of co-pending U.S. patent application Ser. No.14/512,815 filed on Oct. 13, 2014 which in turn is acontinuation-in-part of U.S. patent application Ser. No. 13/803,698,filed on Mar. 14, 2013, now abandoned, which in turn claims priority toU.S. provisional application No. 61/761,669, filed on Feb. 6, 2013. TheSer. No. 14/512,815 application is also a continuation ofPCT/US2014/58824 filed on Oct. 2, 2014. The specifications of everyapplication noted above are incorporated herein by reference.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to resin compositions that include one or morehigh Dk materials in along with a base resin wherein the high Dkmaterial(s) has a dielectric constant is greater than the dielectricconstant of the cured base resin. This invention also relates toprepregs and laminates having a homogeneous dielectric constant acrossthe prepreg or laminate cross sections that are made with the inventiveresin compositions.

(2) Description of the Art

Prepregs and copper clad laminates are planar materials that areroutinely used in the manufacture of printed circuit boards. Prepregsand laminates are typically composite structures that include areinforcing material such as woven glass, non-woven glass, paper, orother fibrous and non-fibrous materials and a polymeric resin that isused as a matrix material—a material that is applied to or used toimpregnate the reinforcing material

With operating frequencies of electronic devices ever increasing, thedielectric properties of the prepregs and laminates are becoming moreimportant to carefully control. One problem with current prepregs andlaminates is that the dielectric properties of the reinforcing materialsand the matrix materials are very different. When very high speedsignals are transmitted through structures such as printed circuitboards built using such metal clad laminates, the signal experiencesskew and a difference in speed as the signal propagates over anisotropicregions. This problem is further compounded when a different signal isrun and in a worst-case scenario, the difference in propagation speedover long lines leads to major signal integrity problems and in somecases to total signal disappearance. This problem has become a majorconcern for electronic device designers especially with onboardfrequencies moving to 14 GHz and beyond to transmit over 100Gigabits/second over four channels in which the skew is expected to be amajor design challenge.

SUMMARY OF THE INVENTION

The present invention is directed to prepregs and laminates that solvethe skew problem by eliminating the gap between the dielectric constantof the matrix and the dielectric constant of the reinforcingmaterial(s). Thus, one aspect of this invention are resin compositionscomprising one or more base resins and one or more high Dk materialswherein the one or more high Dk materials are present in the resincomposition in an amount sufficient to impart the resin composition witha cured Dk that matches the Dk of the reinforcing material to which theresin composition is applied to within plus or minus (±) 15%.

Another aspect of this invention are resin compositions comprising atleast one base resin and from about 5 to about 60 wt % of particles ofor more high Dk materials selected from the group consisting ofstrontium titanate, barium titanate, lead titanate, lead zirconatetitanate, lead lanthanum zirconate titanate and combinations thereofwherein the Dk of the resin composition matches the Dk of a woven glassfabric reinforcing material to which the resin composition is applied towithin plus or minus (±) 15%.

Still another aspect of this invention are prepregs comprising areinforcing material having a Dk_(R), and a resin composition includingone or more base resins having a Dk_(W) where Dk_(R) is more than 15%greater than Dk_(W) the resin composition further including one or morehigh Dk materials present in the resin composition in an amountsufficient to impart the resin composition with a cured Dk_(W) thatmatches the Dk_(R) of the reinforcing material to which the resincomposition is applied to within plus or minus (±) 15%.

Yet another embodiment of this invention are prepregs comprising: an atleast partially cured resin impregnated reinforcing material portion;and a free resin portion wherein the resin includes at least one highdielectric constant material, the free resin portion having a dielectricconstant DK_(w) and the resin impregnated reinforcing material portionhaving a dielectric constant DK_(WR) wherein the high dielectricconstant material is incorporated into the resin in an amount sufficientto match the DK_(WR) with the DK_(W) such that the DK_(WR) and DK_(W)differ by no more than (±) 15%.

DESCRIPTION OF THE FIGURES

FIG. 1 is a cross-section of a prepreg or laminate embodiment of thisinvention that includes a homogeneous dielectric constant across itscross section.

DESCRIPTION OF CURRENT EMBODIMENTS

This invention is directed generally to reinforced prepregs andlaminates used in the electronics industry that include a resin ormatrix component and a reinforcing component. The starting componentsused in the prepregs and laminates of this invention have dielectricconstants that differ by more than 15%. In particular, the resincomponent has a DK_(W) and the reinforcing component has a DK_(R) andthe DK_(W) and the DK_(R) differ by more than 15%. Indeed, it is commonfor the DK_(R) and DK of the resin material without a high Dk material(DK₀) to easily differ by more than 30%

The term “starting component dielectric constant” refers to thedielectric constant of each starting material before the startingmaterials are combined to form a resin impregnated reinforcing materialand/or before they are incorporated into a reinforced prepreg and/orlaminate.

In one embodiment of this invention, the resin component is modifiedwith a high dielectric constant material to form a prepreg or laminatein which the matrix component dielectric constant and the reinforcingmaterial dielectric constant are “homogeneous” or “match. The term“homogeneous” or “match” are used similarly herein to refer to twodielectric constants that differ from one another by no more than plusor minus (±) 15% and more preferably by no more than plus or minus (±)5%.

Another embodiment of this invention are prepregs and laminates havingmatching or homogeneous dielectric constant across their cross section.With a prepreg or laminate, this means that the dielectric constant of aresin impregnated reinforcing material portion (DK_(WR)) of the prepregor laminate matches the dielectric constant of a free resin portion(DK_(W)) of the prepreg or laminate. In this embodiment, the dielectricconstant of the resin is matched with the dielectric constant of theresin impregnated reinforcing material to form a laminate with amatching dielectric constant across its cross section.

In the embodiments above, the dielectric constant of the reinforcingmaterial(s) (Dk_(R)) is generally fixed. Moreover, the dielectricconstant of the resin or matrix material without the high dielectricconstant material (Dk₀) is generally significantly different, i.e.,greater than ±15% different, from the dielectric constant of thereinforcing material (Dk_(R)). Therefore, this invention matchesdielectric constants by incorporating one or more high dielectricconstant (Dk) materials into the resin (also referred to as the matrixmaterial) before associating matrix material with the reinforcingmaterial.

Referring now to FIG. 1 there is shown a prepreg (10) of this inventionhaving a cross section “Y”. The prepreg includes a free resin portion(12) and a resin impregnated reinforcing material portion (14). The freeresin portion has a Dk_(W). The resin impregnated reinforcing materialportion has a Dk_(WR). The free resin portion of the prepreg or laminateincludes any resin having a Dk_(W) that can be determined independentlyof the Dk_(WR). The term “free resin” refers to resin that is part ofthe prepreg or laminate but that is at least 1 micron from anyreinforcement surface—top (16) bottom (18) or sides (20) and (22)—afterthe resin is incorporated into the resin impregnated reinforcingmaterial portion. The free resin will generally be a b-staged orc-staged resin. This includes for example free resin that remains aftera resin coated copper sheet is applied resin down to a core resinimpregnated reinforcing material to form a copper coated prepreg orlaminate.

The “dielectric constants” discussed herein and the dielectric constantranges or numbers referred to herein are determined by the Bereskin testmethod or, in the alternative, by the split post method. Where acomparison of dielectric constants is discussed, then the compareddielectric constants are determined by the same test method. Thedielectric constant of the resin is determined using a fully cured resinsample. The dielectric constant of the resin impregnated reinforcingmaterial portion Dk_(WR) is determined using a sample of a fully curedresin impregnated reinforcing material.

The reinforcing material may be any sheet or ground material(s) that areknown to be useful in manufacturing substrate sheets for fabricating aprepregs or laminates used to manufacture printed circuit boards. While,as noted, ground materials such as ground glass fiber materials may beused, it is preferred that the reinforcing material is a sheet material.For example, the reinforcing sheet material may be inorganic fiber clothincluding various glass cloth (e.g., roving cloth, cloth, a chopped mat,and a surfacing mat), metal fiber cloth, and the like; woven or unwovencloth made of liquid crystal fiber (e.g., wholly aromatic polyamidefiber, wholly aromatic polyester fiber, and polybenzazole fiber); wovenor unwoven cloth made of synthetic fiber (e.g., polyvinyl alcohol fiber,polyester fiber, and acrylic fiber); natural fiber cloth (e.g., cottoncloth, hemp cloth, and felt); carbon fiber cloth; and natural cellulosiccloth (e.g., craft paper, cotton paper, and paper-glass combined fiberpaper).

In one aspect of the invention, the reinforcing material is a wovenglass fabric material. Such woven glass fabric materials will have aDk_(R) of from about 3.5 to 7.0 or greater. Examples of such woven glassfabric materials include, for example, low Dk glass having a Dk_(R) offrom about 3.5 to about 4.5, E-glass; R-glass, ECR-glass, S-glass,C-glass, Q-glass and any other woven glass fabric of the kind known tobe useful in preparing glass fabric reinforced prepregs and laminates.

The resin compositions of this invention will include one or more baseresins that are known in the art to be useful in manufacturing prepregand laminate materials. The base resin will typically be a thermoset orthermoplastic resin Examples of useful base resins include epoxy resins,polyphenylene ether based resins, cyanurate resins, bismaleimide resins,polyimide resins, phenolic resins, furan resins, xylene formaldehyderesins, ketone formaldehyde resins, urea resins, melamine resins,aniline resins, alkyd resins, unsaturated polyester resins, diallylphthalate resins, triallyl cyanurate resins, triazine resins,polyurethane resins, silicone resins and any combination or mixturethereof.

In one aspect of this invention, the base resin is or includes an epoxyresin. Some examples of useful epoxy resins include phenol types such asthose based on the diglycidyl ether of bisphenol A, on polyglycidylethers of phenol-formaldehyde novolac or cresol-formaldehyde novolac, onthe triglycidyl ether of tris(p-hydroxyphenol)methane, or on thetetraglycidyl ether of tetraphenylethane; amine types such as thosebased on tetraglycidyl-rnethylenedianiline or on the triglycidyl etherof p-aminoglycol; cycloaliphatic types such as those based on3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate. The term“epoxy resin” also refers to reaction products of compounds containingan excess of epoxy (for instance, of the aforementioned types) andaromatic dihydroxy compounds. These compounds may behalogen-substituted. Preference is given to epoxy-resins which arederivative of bisphenol A, particularly FR-4. FR-4 is made by anadvancing reaction of an excess of bisphenol A diglydicyl ethertetrabromobisphenol A. Mixtures of epoxy resins with bismaleimide resin,cyanate resin and/or bismaleimide triazine resin can also be applied.

The resin compositions, in addition to the base resin will typicallyinclude initiators or catalysts, one or more optional flame retardantsand solvents. The flame retardant may be any flame retardant materialthat is known to be useful in resin compositions used to manufactureprepregs and laminates use to manufacture printed circuit boards. Theflame retardant(s) may contain halogens or they may be halogen free.Alternatively or in addition, the resins may include halogens such asbromine to impart the cured resin with flame retardant properties.

The resin composition may also include polymerization initiators orcatalysts. Examples of some useful initiators or catalysts include, butare not limited to peroxide or azo-type polymerization initiators(catalysts). In general, the initiators/catalysts chosen may be anycompound that is known to be useful in resin synthesis or curing whetheror not it performs one of these functions.

The resin composition will include one or more solvents which aretypically used to solubilize the appropriate resin compositioningredients and/or to control resin viscosity and/or in order tomaintain the resin ingredients in a suspended dispersion. Any solventknown by one of skill in the art to be useful in conjunction withthermosetting resin systems can be used. Particularly useful solventsinclude methylethylketone (MEK), toluene, dimethylformamide (DMF), ormixtures thereof. As noted below, the resin compositions are used tomanufacture prepregs and laminates. During the manufacturing process,the reinforcing materials are impregnated with or otherwise associatedwith the resin compositions and some more most of the solvent is removedfrom the resin compositions to form the prepregs and laminates. Thus,when resin composition weight percent amounts are listed, they arereported on a dry-solvent free-basis unless otherwise noted.

The resin compositions may include a variety of other optionalingredients including fillers, tougheners, adhesion promoters, defoamingagents, leveling agents, dyes, and pigments. For example, a fluorescentdye can be added to the resin composition in a trace amount to cause alaminate prepared therefrom to fluoresce when exposed to UV light in aboard shop's optical inspection equipment. Other optional ingredientsknown by persons of skill in the art to be useful in resins that areused to manufacture printed circuit board laminates may also be includedin the resin compositions of this invention.

The resin compositions of this invention will also include one or morehigh Dk materials. The high Dk materials can be any materials that canbe incorporated into a liquid resin such that the Dk of the cured orpartially cured resin composition including the high Dk material isdifferent from and preferably higher than the Dk of the resincomposition resin ingredient(s). In one embodiment, the high Dk materialwill have a Dk of greater than about 200 and more preferably greaterthan about 500.

One class of useful high Dk materials are ferroelectric materials.Examples of some useful ferroelectric materials include strontiumtitanate, barium titanate, lead titanate, lead zirconate titanate, leadlanthanum zirconate titanate and combinations thereof. Particularlyuseful high DK materials are strontium titanate and barium titanate.

The high DK materials can be incorporated into the resin compositions asa particulate material. If a particular material is used, then the highDK material will typically have particle sizes ranging from about 1 nmto 40 microns.

The high DK material will be included in the resin composition in anamount sufficient to form a homogeneous prepreg or laminate. In oneembodiment, a homogeneous prepreg or laminate will have a resincomposition or matrix Dk_(W) that is within ±15% of the reinforcingmaterial Dk_(R) and preferably within ±5%. In an alternative,embodiment, the homogeneous prepreg or laminate will have a free resinportion that has a Dk_(W) that is within ±15% of the resin impregnatedreinforcing material portion Dk_(WR) and preferably within ±5%.

The amount of high DK material that is incorporated into the resincomposition will vary depending upon the DK₀ of the base resin and aDK_(R) of the reinforcing material. Generally, the greater thedifference between the Dk₀ and Dk_(R) the greater the amount of high Dkmaterial that will be included in the resin composition. Generally, anamount of high Dk material that is greater than about 2 wt % of theresin composition on a dry basis is necessary to cause a change in thebase resin Dk. The maximum amount of high Dk material that can beincorporated into the resin composition without significantly impactingresin composition properties is about 70 wt % on a dry, solvent freebasis. In an alternate embodiment the high Dk material will be presentin the resin composition in an amount ranging from about 5 to about 60wt % on a dry basis. We have discovered that adding from about 5 toabout 60 wt % of particulate barium titanate to a base resin having aDk₀ of about 4 increases the DkW of the resin composition from justabove 4 at a 5 wt % loading to higher than 7.5 at a 60 wt % loading.

The resin compositions described above are especially useful forpreparing prepregs and/or laminates used in the manufacture of printedcircuit boards. In order to be useful in manufacturing printed circuitboards the laminates can be partially cured or b-staged—to form what isknown in the industry as a prepreg—in which state they can be laid upwith additional material sheets to form a c-staged or fully curedlaminate sheet. Alternatively, the resins can be manufactured intoc-staged or fully cured material sheets.

In one useful processing system, the resin composition/reinforcingmaterial combinations described above are useful for making prepregs ina batch or in a continuous process. Prepregs are generally manufacturedusing a core material such as a roll of woven glass web (fabric) whichis unwound into a series of drive rolls. The web then passes into acoating area where the web is passed through a tank which contains thethermosetting resin system of this invention, solvent and othercomponents where the glass web becomes saturated with the resin. Thesaturated glass web is then passed through a pair of metering rollswhich remove excess resin from the saturated glass web and thereafter,the resin coated web travels the length of a drying tower for a selectedperiod of time until the solvent is evaporated from the web. A secondand subsequent coating of resin can be applied to the web by repeatingthese steps until the preparation of the prepreg is complete whereuponthe prepreg is wound onto roll The woven glass web can replaced with awoven fabric material, paper, plastic sheets, felt, and/or particulatematerials such as glass fiber particles or particulate materials.

In another process for manufacturing prepreg or laminate materials,thermosetting resins of this invention are premixed in a mixing vesselunder ambient temperature and pressure. The viscosity of the pre-mix is˜600-1000 cps and can be adjusted by adding or removing solvent from theresin. Fabric substrate—such as E glass—is pulled through a dip tankincluding the premixed resin, through an oven tower where excess solventis driven off and the prepreg is rolled or sheeted to size, layed upbetween Cu foil in various constructions depending on glass weave style,resin content & thickness requirements.

The resin composition can also be applied in a thin layer to a Cu foilsubstrate (RCC—resin coated Cu) using slot-die or other related coatingtechniques.

The resins, prepregs and resin coated copper foil sheets described abovecan be used to make laminates, such as those used to manufacture printedcircuit boards, in batch or in continuous processes. In exemplarycontinuous process for manufacturing laminates of this invention, acontinuous sheet in the form of each of copper, a resin prepreg and athin fabric sheet are continuously unwound into a series of drive rollsto form a layered web of fabric, adjacent to the resin prepreg sheetwhich is adjacent to a copper foil sheet such that the prepreg sheetlies between the copper foil sheet and the fabric sheet. The web is thensubjected to heat and pressure conditions for a time that is sufficientto cause the resin to migrate into the fabric material and to completelycure the resin. In the resulting laminate, the migration of the resinmaterial into the fabric causes the thickness of the resin layer (thedistance between the copper foil material and the fabric sheet materialto diminish and approach zero as combination layers discussed abovetransforms from a web of three layers into a single laminate sheet. Inan alternative to this method, a single prepreg resin sheet can beapplied to one side of the fabric material layer and the combinationsandwiched between two copper layers after which heat and/or pressure isapplied to the layup to cause the resin material to flow and thoroughlyimpregnate the fabric layer and cause both copper foil layers to adhereto the central laminate.

In still another embodiment, resin composition coated copper sheets canbe made at the same time the laminate is being made by applying a thincoating of resin to two different continuously moving copper sheets,removing any excess resin from the sheets to control the resin thicknessand then partially curing the resin under heat and/or pressureconditions to form a sheet of b-staged resin coated copper. The sheet(s)of b-staged resin coated copper can then be used directly in thelaminate manufacturing process.

In yet another embodiment, the fabric material—with or without priorpretreatment—can be continuously fed into a resin composition bath suchthat the fabric material becomes impregnated with the resin composition.The resin composition can be optionally partially cured at this stage inthe process. Next, one or two copper foil layers can be associated withthe first and/or second planar surface of the resin compositionimpregnated fabric sheet to form a web after which heat and/or pressureis applied to the web to fully cure the resin composition.

The invention has been described in an illustrative manner. It is to beunderstood is that the terminology, which has been used, is intended tobe in the nature of words of description rather than limitation. Manymodifications and variations of the invention are possible in light ofthe above teachings. Therefore, within the scope of the appended claims,the invention may be practiced other than as specifically described.

What is claimed is:
 1. A prepreg comprising: an at least partially curedresin impregnated reinforcing material portion; and a free resin portionwherein the resin includes at least one high dielectric constantmaterial, the free resin portion having a dielectric constant DKw andthe resin impregnated reinforcing material portion having a dielectricconstant DK_(WR) wherein the high dielectric constant material isincorporated into the resin in an amount sufficient to match the DK_(WR)with the DK_(w) such that the DK_(WR) and DK_(W) differ by no more than(±) 15%.
 2. The prepreg of claim 1 wherein the one or more high Dkmaterials each have a Dk of at least
 500. 3. The prepreg of claim 1wherein the one or more high Dk materials are particulate materials. 4.The prepreg of claim 3 wherein the one or more high Dk material particlesize ranges from about 1 nm to about 40 microns.
 5. The prepreg of claim1 wherein the one or more high Dk materials are ferroelectric materials.6. The prepreg of claim 5 wherein the ferroelectric materials areselected from the group consisting of strontium titanate, bariumtitanate, lead titanate, lead zirconate titanate, lead lanthanumzirconate titanate and combinations thereof.
 7. The prepreg of claim 1wherein the base resin is a thermoset or thermoplastic resin.
 8. Theprepreg of claim 1 wherein one or more high Dk materials are present inthe composition in an amount ranging from about 2 to about 70 wt %. 9.The prepreg of claim 1 wherein the reinforcing material is selected fromthe group consisting of woven glass fabric, paper, felt, glass fibersand plastic sheets.
 10. The prepreg of claim 9 wherein the reinforcingmaterial is a low Dk woven glass fabric.
 11. The prepreg of claim 10wherein the low Dk woven glass fabric has a Dk ranging from about 3.5 toabout 7.0.
 12. The prepreg of claim 1 wherein the free resin is anyresin that is at least 1 micron from any reinforcement surface.
 13. Aprepreg of claim 1 wherein the resin composition comprises at least onebase resin and from about 5 to about 60 wt % of particles of or morehigh Dk materials selected from the group consisting of strontiumtitanate, barium titanate, lead titanate, lead zirconate titanate, leadlanthanum zirconate titanate and combinations.
 14. The prepreg of claim1 wherein the DK_(WR) and DK_(W) differ by no more than (±) 5%.
 15. Alaminate including a fully cured prepreg of claim
 1. 16. The laminate ofclaim 15 including at least one copper layer.
 17. A printed circuitboard including as at least one layer, a fully cured prepreg of claim 1.